Methods and systems for characterizing laser machining properties by measuring keyhole dynamics using interferometry

ABSTRACT

A method, apparatus, and system are provided to monitor and characterize the dynamics of a phase change region (PCR) created during laser welding, specifically keyhole welding, and other material modification processes, using low-coherence interferometry. By directing a measurement beam to multiple locations within and overlapping with the PCR, the system, apparatus, and method are used to determine, in real time, spatial and temporal characteristics of the weld such as keyhole depth, length, width, shape and whether the keyhole is unstable, closes or collapses. This information is important in determining the quality and material properties of a completed finished weld. It can also be used with feedback to modify the material modification process in real time.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 16/383,544,filed Apr. 12, 2019, now U.S. Publication No. 2019-0299327 dated Oct. 3,2019, which is a continuation of application Ser. No. 15/684,145, filedAug. 23, 2017, now U.S. Pat. No. 10,413,995 dated Sep. 17, 2019, whichis a continuation of application Ser. No. 14/775,136, filed Sep. 11,2015, now U.S. Pat. No. 9,757,817 dated Sep. 12, 2017, which is a 371 ofInternational No. PCT/CA2014/000273, filed Mar. 13, 2014, which claimsthe benefit of U.S. Provisional Application No. 61/778,592, filed onMar. 13, 2013, hereby incorporated herein by reference in its entirety.

FIELD

This invention relates to imaging using interferometry, includinglow-coherence interferometry, and to optical modification or measurementof materials, such as through the use of lasers in processes such asmachining and welding.

BACKGROUND

Lasers are known to be important tools for processing a wide range ofmaterials. In particular, lasers are very well suited to and see wideapplication for processing of metals, polymers, ceramics,semiconductors, composites and biological tissue. By focusing a laserbeam, it can be possible to achieve improved precision of the laser'saction in a direction transverse to the beam axis. However, localizingthe laser's action in the axial direction of the beam can be difficult.During processes such as laser welding, a phase change region (PCR) iscreated where the material localized to the bonding region changesdynamically from solid to a liquid and/or a gas state and back to asolid again at the completion of the weld. In some cases the materialmay change multiple times between the various states and also interactwith other substances present in the weld zone including other solids,liquids and gasses. Controlling this phase change region (PCR) isimportant to control the quality of the weld and the overallproductivity of the welding system. The high spatial coherence of laserlight allows good transverse control of the welding energy deposition,but thermal diffusion limits the achievable aspect ratio of weldedfeatures when the energy is transmitted through the material withconduction alone. For higher aspect ratio features, the more dynamic andunstable process of keyhole welding is used to allow the conversion ofoptical to thermal energy to occur deeper in the material. Here, axialcontrol (depth of the PCR) is even more problematic. In keyhole welding,the depth of the PCR and the absorption of the laser may extend deepinto the material (for example, depths from 10 micrometers to tens ofmillimeters). Here, the beam intensity is sufficient to melt the surfaceto open a vapor channel (also known as a capillary or “the keyhole”)which allows the optical beam to penetrate deep into the material.Depending on the specific application, the keyhole may be narrow (e.g.,less than 1 mm) but several millimeters deep and sustained with theapplication of optical power (for example in the range from 1-2 W to20,000 W or more). As a result, the light-matter interaction regioninside the PCR can be turbulent, unstable and highly stochastic.Unfortunately, instability of keyhole formation can lead to internalvoids and high weld porosity resulting in weld failure, with potentialcatastrophic consequences. Similarly, keyhole instability can result inspatter that contaminates nearby system components, complicating theapplication of laser welding in systems such as vehicular transmissions.Weld quality verification is usually required, often using expensiveex-situ and destructive testing. Welding imaging solutions are offeredbut are limited in their capabilities and usually monitor regions eitherbefore or after the PCR, to track the weld joint, or record the topsurface of the cooled weld joint.

SUMMARY

Some embodiments of the invention involve characterization ofmorphology, for example, including one or more of length, width, depth,size, shape, and aspect ratio of the keyhole and surrounding materialover time by directing an interferometry measurement beam (including,for example, a low-coherence interferometry measurement beam) into thePCR and surrounding area. The beam may be moved along an x- or y-axisand/or θ/φ (i.e., theta/phi, angle may change from normal).

According to one aspect of the invention, there is provided an apparatuscomprising: an imaging optical source that produces imaging light thatis applied to a material processing system, wherein the materialprocessing system implements a material modification process and createsa phase change region (PCR) in a material; at least one element thatdirects the imaging light at a plurality of imaging beam positionsproximate the PCR; at least one input-output port that outputs a firstcomponent of the imaging light to an optical access port of the materialprocessing system and that receives a reflection component of theimaging light; an optical combiner that combines the reflectioncomponent and at least another component of the imaging light to producean interferometry output, the interferometry output based on a pathlength taken by the first component and the reflection componentcompared to a path length taken by the at least another component of theimaging light; and an interferometry output processor that processes theinterferometry output to determine at least one characteristic of thePCR.

In some embodiments the apparatus may further comprise a materialprocessing beam source that produces a material processing beam that isapplied to the material in the material modification process, whereinthe material processing beam creates the PCR in the material.

According to another aspect of the invention, there is provided anapparatus for modifying a sample, the apparatus comprising: a materialprocessing beam source that produces a material processing beam that isapplied to the sample at a sample location in a material modificationprocess wherein the material processing beam creates a phase changeregion (PCR) in the sample; an imaging optical source that producesimaging light that is applied at a plurality of imaging beam positionsproximate the PCR (i.e., in the vicinity of the PCR and/or within thePCR); an optical interferometer that produces an interferometry outputfor each imaging beam position using at least a component of the imaginglight that is delivered to the sample, the interferometry output basedon at least one optical path length to the sample compared to anotheroptical path length; and an interferometry output processor thatprocesses the interferometry outputs to determine at least onecharacteristic of the PCR.

According to another aspect of the invention, there is provided anapparatus for use with a material processing system that implements amaterial modification process and creates a phase change region (PCR) ina material, the material processing system having an optical accessport, the apparatus comprising: an imaging optical source that producesimaging light that is applied at a plurality of imaging beam positionsproximate the PCR; at least one input-output port that outputs a firstcomponent of the imaging light to the optical access port of thematerial processing system and that receives a reflection component ofthe imaging light; an optical combiner that combines the reflectioncomponent and at least another component of the imaging light to producean interferometry output, the interferometry output based on a pathlength taken by the first component and the reflection componentcompared to a path length taken by the at least another component of theimaging light; and an interferometry output processor that processes theinterferometry outputs to determine at least one characteristic of thePCR.

According to another aspect of the invention, there is provided a methodcomprising: applying an imaging light to a material processing system,wherein the material processing system implements a materialmodification process and creates a phase change region (PCR) in amaterial; using at least one element to direct the imaging light at aplurality of imaging beam positions proximate the PCR; outputting afirst component of the imaging light to an optical access port of thematerial processing system and receiving a reflection component of theimaging light; combining the reflection component and at least anothercomponent of the imaging light to produce an interferometry output, theinterferometry output based on a path length taken by the firstcomponent and the reflection component compared to a path length takenby the at least another component of the imaging light; and processingthe interferometry output to determine at least one characteristic ofthe PCR.

In some embodiments, the method may further comprise applying a materialprocessing beam to the material in the material modification process,wherein the material processing beam creates the PCR in the material.

According to another aspect of the invention, there is provided a methodfor modifying a sample, the apparatus comprising: producing a materialprocessing beam that is applied to a sample at a sample location in amaterial modification process wherein the material processing beamcreates a phase change region (PCR) in the sample; producing imaginglight that is applied at a plurality of imaging beam positions proximatethe PCR; producing an interferometry output for each imaging beamposition using at least a component of the imaging light that isdelivered to the sample, the interferometry output based on at least oneoptical path length to the sample compared to another optical pathlength; and processing the interferometry outputs to determine at leastone characteristic of the PCR.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described below, by way of example, with referenceto the accompanying drawings, wherein:

FIG. 1 is a cross section diagram of a material welding processfeaturing keyhole imaging in accordance with an embodiment of theinvention;

FIG. 2 is a schematic diagram of an apparatus that implements keyholeimaging in a material welding process, according to one embodiment;

FIG. 3 is a schematic diagram of another apparatus that implementskeyhole imaging in a material welding process, according to anotherembodiment, similar to the apparatus used to generate the images inFIGS. 4A-4E;

FIGS. 4A-4E depict experimental keyhole imaging image data obtainedduring welding using a 1.1 kW laser on the sample, a 200 um weldingspot, a ˜70 um imaging spot, at 20 mm/s, and imaging sample rate of 100kHz, wherein the imaging beam was leading (FIG. 4A), aligned with (FIG.4B), or trailing (FIGS. 4C, 4D, 4E) the processing beam;

FIG. 5 shows an example of an image of a laser spot weld;

FIG. 6 shows an example of a system with an adjustable delay line in thereference arm;

FIG. 7 shows an example of a system with separate objectives for theprocessing beam and the image beam;

FIGS. 8 and 9 show two example interferometry systems;

FIG. 10 shows two images of lap welding with digitally tracked keyholefloors, further showing examples of keyhole instability;

FIG. 11 shows experimental interferometry data from the PCR of a laserweld at a plurality of positions ranging from in front of the processingbeam to behind the processing beam;

FIG. 12 shows experimental interferometry data from the PCR of a laserweld at a plurality of positions ranging from the left to the right ofthe processing beam;

FIGS. 13A-13D are coherent images of keyhole laser welding with theimaging beam aligned ahead of or behind the processing beam; and

FIGS. 14A and 14B are schematic diagrams of further embodiments of anapparatus that implements keyhole imaging in a material welding process,using a pre-objective scanner (FIG. 14A) or a post-objective scanner(FIG. 14B).

DETAILED DESCRIPTION

In all embodiments described herein, a material modification beam, alsoreferred to as a material processing beam, is used. Examples of amaterial processing beam include a laser beam, an electron (or otherparticle) beam, plasma beam, electric arc, or water jet. Auxiliary laserbeams and combinations of these (e.g., a laser beam guided by a waterjet, hybrid laser arc welding) are also encompassed. Thus, whereas mostembodiments are described as using a laser beam, it will be understoodthat the invention is not limited thereto.

As used herein, the terms “keyhole”, “capillary”, and “vapour channel”are considered to be equivalent and are intended to refer to the gaseouscavity that exists in a phase change region of a material during amaterial modification process using a material modification beam.

FIG. 1 is a cross section diagram of a typical material welding processfeaturing coherent imaging in accordance with an embodiment of theinvention. Two metal samples 12 and 14 are to be joined together in acontinuous welding (CW), keyhole welding laser process. The laser beam16 is moved across the surface in the direction indicated by arrow 17.

The PCR (phase change region) comprises a liquid region 32, a gas orkeyhole region 30, and a bonded solid region 34, the solid having beenreformed from the other two states. In general, if keyhole welding isoccurring successfully, there will be three phases, as depicted inFIG. 1. However, in some embodiments, the apparatus and method are usedto detect the lack of keyhole formation, in which case there may be onlyliquid and solid states, or only a solid state.

A plurality of imaging beams 20 (herein depicted as 20 a through 20 i)are introduced at multiple points and/or at multiple incident angles in,and optionally near, the PCR. In the specific example depicted, thereare seven beams 20 a, 20 b, 20 c, 20 d, 20 e, 20 f, 20 g that aresubstantially normal to samples, and two beams 20 i, 20 h that haveincident angles that are not normal to the samples. The imaging beams 20are used to generate measurements using low-coherence interferometry ateach of the multiple points and/or multiple incident angles. While FIG.1 shows a specific plurality of imaging beams 20 introduced at aspecific set of points and incident angles, more generally, a pluralityof measurements at some set of imaging beam positions are taken. Themultiple imaging beam positions may involve one or a combination of:

-   -   one or more static beams;    -   one or more beams that are moved;    -   one or more beams normal to the sample location;    -   one or more beams whose angle is changed;    -   one or more beams that are moved and whose angles are changed;        and    -   beams that originate from one or multiple light sources,        including a light source that is multiplexed to produce multiple        outputs.

In some embodiments, one of the plurality of imaging beam positions iscreated by the multiple internal reflection of an imaging beam inside ofan optical element (which may also be referred to herein simply as an“optic”) that the imaging light interacts with inside of the beamdelivery system. This multiple reflection introduces additional opticalpath length (thus shifting the location of the reflection to a depth inthe image and allowing it to be distinguished from the another beammeasuring something else such as the keyhole depth) and a transverseshift of the focus of the beam. This allows for convenient simultaneousmeasurement of the top surface reference point(s) (TSRP) and weld depth.Top surface reference points are discussed in further detail below. Animage showing such simultaneous measurement capability is shown in FIG.4B. The reflection showing the TSRP is located at an indicated depth ofapproximately 650 μm.

Interferometry/Coherent Imaging Implementation

Each of a plurality of imaging beams (e.g., beams 20 a-20 i) originatesfrom a semi-coherent light source, although as described above multiplebeams may originate from a single light source. A very specific exampleof this type of light source is a superluminescent diode with a spectrumranging from 820-860 nm and output power of 20 mW coupled into a singlemode optical fiber such as Corning HI780. Light sources meeting thesecriteria are commercially available and manufactured by Superlum DiodesInc. (Ireland) and other manufacturers. The beam from the light sourceis carried, directed and manipulated through various media andcomponents that might include fiber optic cables, air (or other gases),mirrors (or semi-reflective mirrors), lenses, or other optics. The fiberoptic cables can be of the single-mode, multimode, and/or polarizationmaintaining types. The light source beam is split into two or morebeams, for example using a semi-reflective mirror. One beam known as theimaging beam or sample beam is directed towards the sample; each of thebeams depicted in the figure as 20 a to 20 i is such a sample beam.Another beam known as the reference beam is reflected off a referencesurface (e.g., a mirror). The sample beam and the reference beam arethen optically recombined, for example by the same semi-reflectivemirror, so that they create and interference pattern. While aMichelson-style interferometer was just described, other interferometerconfigurations such as Mach-Zehnder (including the use of opticalcirculators), Sagnac, and common-path may also be applied in someembodiments. The interference pattern, I(k), will vary depending on thepath length of the reflected imaging beam relative to the path length ofthe reference beam, Δz, according to the relationshipI(k)=A(k)└I(k)_(reference)+I(k)_(sample)+(√{square root over(I(k)_(sample)·I(k)_(reference))})cos(2kΔz)┐. These interferometrypatterns are then captured and digitized using a commercially availablespectrometer and camera such as the DeepView™ NIR spectrometer (BaySpec,Inc. San Jose, USA). Additional established optical coherence tomographytechniques and those from inline coherent imaging are then used tocalculate depth relative to a known reference position. Specificexamples of interferometry systems will be described below withreference to FIGS. 8 and 9.

Calculation of Keyhole/PCR Characteristics and/or Parameters

The following are examples of methods that may be used calculate keyholePCR characteristics and/or parameters. A reference position(s) isestablished using points on the sample surface identified to be TSRPs.In the case where the sample is substantially flat, at least one TSRPcan be used to define a top surface reference plane. Additional topsurface reference points can be determined based on the top surfacereference plane without taking corresponding additional measurements.Alternatively, multiple top surface reference points are used tocalculate depth of the process.

The reference position, such as the TSRP, may be set, measured, orcalibrated before, during, or after the welding process. This may beachieved by taking a baseline depth measurement or measurements atlocations on the sample unaffected by the welding process, such as thelocation illuminated by beam 20 g in FIG. 1. The TSRP can also bedefined in real time by simultaneously imaging the top surface andkeyhole bottom either through the use of multiple imaging channels or byenlarging the imaging spot to simultaneously or dynamically (i.e.,sometimes the top, sometimes the bottom based on keyhole oscillations)cover both locations. In the simplest case, the TSRP can be determinedby taking one or more measurements of the material immediately beforethe weld begins. If the material is sufficiently flat relative to theweld motion, then this initial measurement can define the TSRP for therest of the weld. In other cases the TSRP is mechanically fixed at aspecific distance or may be measured using other standard electrical,mechanical, or optical means. An example of this would be a beamdelivery system that rolls across the workpiece(s). In this case, thevirgin material surface would be a known distance away from the weldingoptics that is directly related to the distance between the unit'swheel(s) and the optics. Another example would be a welding system thatutilizes a fixture or clamp to hold the workpiece(s). Again, since thedistance between the optics and the fixture is known, the distancebetween the optics and the virgin surface of the material is known.

The imaging beams, such as beams 20 of FIG. 1, are used to measure,instantaneously and/or over a period of time, one or a combination oftwo or more of keyhole length, width, depth, surface shape, sub-surfaceprofile, wall slope, collapse, instability, undercut, and other physicalparameters of the PCR. Specific example methods of calculating each ofthese values will now be described. More generally, what constituteslength, width, depth, surface shape, sub-surface profile, wall slope,collapse, instability, undercut, or the other physical parameters of thePCR can be defined on an implementation-specific basis.

A single depth measurement is generally defined as the distance belowthe TSRP measured by the imaging beam. Note that depth can be a negativevalue if the measurement is above the TSRP.

For the following examples, the imaging beams are normal or close tonormal to the sample surface.

Keyhole Depth—

Keyhole depth for any instant in time is generally defined as thedeepest point of the keyhole. This may be, for example, by takingmultiple depth measurements within the keyhole and taking the maximum ofthese readings. Because the keyhole changes over time, in someembodiments, readings are taken in succession to determine how maximumdepth changes over time. In practice, due to material properties anddepth accuracy required, only a limited number of measurements in bothposition and time may be necessary. In other cases, a large number ofmeasurements locations and/or measurement at high speed may beperformed.

Location of Maximum Keyhole Depth—

The location of maximum keyhole depth is the location at any instance intime from which the keyhole depth value is determined (i.e., the deepestlocation).

Average Keyhole Depth—

The average keyhole depth is determined by taking the average of theindividual keyhole depth values over some period of time. Otherstatistical techniques (e.g., standard deviation, median, min/maxthresholds, higher order moments) may also be applied. Such statisticaltechniques can be used as direct indicators of weld stability, theprobability of defects and therefore quality. Statistical snapshots ofweld regions produced by image processors may also be used byfeedback/process controllers to trigger annunciations and effect changesto weld parameters.

Keyhole Location—

the relative positions of the leading edge, trailing edge, left side andright side of the keyhole relative to the processing (e.g., laser) beam.

Keyhole Length—

Keyhole length is determined, for example, by calculating the furthestdistance between two measurement beam readings that are below the TSRPand aligned with the axis of laser motion. For example, in FIG. 1, thekeyhole length might be defined by the distance between measurementbeams 20 a and 20 d.

Keyhole Width—

Keyhole width may be similarly defined but with readings alignedperpendicular to the axis of laser motion.

Keyhole Surface Shape—

The left and right side widths of the keyhole as measured relative tothe processing laser at various points along the length of the keyhole.

Subsurface Keyhole Length and Width—

Subsurface keyhole length and/or width can also be determined bycalculating the length and/or width values relative to a plane at apredetermined distance below the TSRP.

Keyhole Profile—

The depth of the keyhole measured at various points along the length ofthe keyhole.

Keyhole Wall Slope—

Wall slope may be determined by calculating the slope of a line thatfits two or more points on the wall of the keyhole. For example a linejoining depth points 20 a and 20 b will give the slope of the front wallof the key hole. Similarly back and side wall slopes can be calculated.

Keyhole Collapse—

Keyhole collapse can be determined if successive readings of keyholedepth temporarily or intermittently fail to meet or exceed somespecified value.

Keyhole Instability—

Keyhole instability can be determined from the variability of successivekeyhole depth readings.

Other calculations using coherent imaging may also be performed.

All of the examples above rely on imaging beams that are normal orsubstantially normal to the plane of the sample surface. In some casesit may be advantageous to take coherent imaging readings at angles thatare not normal to the plane of the sample surface. Readings from imagingbeams 20 h and 20 i of FIG. 1 would be examples of this. For example, insome embodiments, these off-normal imaging beams are used to determineor contribute to the determination of one or more of wall instability,partial keyhole collapse or situations that could lead to voids orporosity in a welding process. Particularly at high welding speeds atdeeper depths, the keyhole vapour channel may undercut some of theliquid (towards the trailing edge of the weld) such that there is not adirect optical path to the bottom of the keyhole that is also normal tothe material surface. This situation is particularly vulnerable tounstable pathological behaviour and may be detected by comparing signalsfrom imaging beams 20 b and 20 h (or one similarly angled to reach thebottom of a undercut keyhole).

The dynamics of the liquid region of the PCR are examined. This can bedone, for example, by taking multiple imaging beam measurements in andaround where the liquid phase region is expected to be located. For theexample of FIG. 1, multiple imaging beam measurements near imaging beam20 e may be used to look at the slope, changes, waves, or othercharacteristics of the liquid.

In some embodiments, the interface between the liquid/solid region ofthe PCR is located using the measurements. In a specific example,multiple imaging beam measurements are taken in and around where theinterface is expected to be located (for example in and around thelocation of beams 20 e, 20 f of FIG. 1). The liquid will oscillate anddetectably change its position whereas the solid region will be static,thus producing measurable contrast between the two phases.

In some embodiments, waves are excited and generated in the liquidregion of the PCR using acoustic and/or optical energy source techniquesto assist with generating imaging contrast and understanding PCRgeometry, dynamics, and characteristics such as viscosity. For example,an acoustic vibration may be excited in the liquid at a frequency thatis smaller than the imaging sample rate. An imaging beam observing sucha liquid region would be able to measure the phase and amplitude of thegeometric distortion that follows the acoustic vibration, therebyconfirming the liquid state of the point being imaged.

In some embodiments of the invention, at least one of the plurality ofimaging beams positions is outside the PCR. Beams 20 f and 20 g areexamples of this in FIG. 1.

In some embodiments of the invention, light is applied to at least twoof the plurality of imaging beams positions simultaneously. The multipleimaging beams can be generated in this case using multiple beam sources,or by using a single beam source and one or more splitters.

In some embodiments of the invention, light is applied to at least twoof the plurality of imaging beam positions sequentially. Thesequentially applied imaging beams can be generated using multiple beamsources that are activated in sequence, or by using a single beam sourcethat is reconfigured to produce each of the beams in sequence.

In some embodiments of the invention, the plurality of imaging beampositions are achieved by changing the position and/or angle of at leastone imaging beam relative to the processing beam during the weldingprocess.

In some embodiments of the invention, the number of positions where theplurality of imaging beams is applied to the sample is changed duringthe welding process.

In some embodiments of the invention, at least one of the plurality ofimaging beam positions does not have an incident position that is on aline formed by the material processing beam. For example, in FIG. 1,laser beam 16 moves in direction 17 and traces out a path. One or moreof the imaging beams can be applied off this path. This can be used, forexample, to determine keyhole width.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beams positions is used to determine thewidth or diameter of the keyhole when viewed from the same direction asthe material processing (e.g., laser) beam is applied.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is focused to a diameterthat is smaller than the diameter of the laser beam.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is focused to a diameterthat is similar to the diameter of the laser beam.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is focused to a diameterthat is larger than the diameter of the laser beam.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is focused to a diameterthat encompasses the PCR.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is focused to a diameterthat is larger than the PCR.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to takesuccessive readings at a frequency of approximately 10 Hz or more.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to takesuccessive readings at a frequency of approximately 100 Hz or more.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to takesuccessive readings at a frequency of approximately 1 kHz or more.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to takesuccessive readings at a frequency of approximately 10 kHz or more.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to takesuccessive readings at a frequency of approximately 100 kHz or more.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to takesuccessive readings at a frequency of approximately 1 MHz or more.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to determine themaximum depth achieved by the keyhole over a period of time. In someembodiments, this determination is used to control at least oneparameter of the welding process to reduce the number of instances wherewelding will penetrate beyond a specified depth and/or into a specifiedmaterial, including reducing the number of instances to zero.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to determine theminimum depth achieved by the keyhole over a period of time. In someembodiments, this determination is used to control at least oneparameter of the welding process to reduce the number of instances wherewelding fails to penetrate beyond a specified depth and/or into aspecified material, including reducing the number of instances to zero.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to determine theshape and size of the keyhole over time.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to determine ifthe keyhole collapses or fails to maintain a specified depth.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to calculate anoptimal speed for the welding process.

In some embodiments of the invention, imaging light applied to at leastone of the plurality of imaging beam positions is used to calculate anoutput power level for the laser beam.

In some embodiments of the invention, measurements for at least one ofthe plurality of imaging beam positions are processed, output, and fedback to the laser process control system to provide closed loopoperation.

FIG. 2 is a schematic diagram of an example of an apparatus thatimplements the coherent imaging in a material welding process. Amaterial processing laser 17 produces a laser beam that is carried via afiber optic cable 26 and connects through fiber attachment connector 25to a laser head 28 which outputs laser beam 16. Embodiments describedherein involve the use of a laser. The laser beam 16 is collimated byoptic 24 and focused by optic 18 such that keyhole welding is achievedon the sample 12, 14. PCR 30, 32, 34 is depicted. A movable dichroicmirror 19 is shown.

A low-coherence interferometry imaging light source 21 produces imaginglight that is carried via fiber optic cable 27 and connected throughfiber attachment connector 23 to the laser head 28 where the imaginglight is used to form an imaging beam 20. A collimating lens 22 directsthe imaging beam 20 towards the mirror 19. The mirror 19 is actuated bya motorized system (not shown) such that the imaging beam 20 can bedirected to multiple locations within the PCR 30 on the sample 12, 14.

In the illustrated example, the laser and the imaging beam (moregenerally light applied to at least one of the imaging beam positions)are focussed by a common focusing lens 18. In some embodiments of theinvention the imaging beam 20 and laser beam 16 may each have their ownseparate focusing lenses which focus the beams before they are combinedby mirror 19 and then delivered to the sample 12, 14. This may beparticularly useful in some applications as it separates the opticalrequirements for imaging (e.g., multiple, possibly aspheric opticalelements for field distortion reduction) and high power beam deliveryoptical requirements (minimal focal shift and power absorption). It alsomakes it easier to position a scanning device near the back focalposition of a scanning lens, which is often desired for reducing opticalpath distortion when using telecentric scanning optics.

The scanning device is used to change the angle and/or position of theimaging beam and/or the processing beam. Examples of a scanning deviceare a galvanometer-mirror scanner (e.g., available from SCANLAB AG inGermany), a piezoelectrically actuated mirror, and a motor actuatedmirror. A scanning lens is an optical device that is used in conjunctionwith or as a part of the scanning device to direct an optical beam. Anexample of a scanning lens is an LSMO5 from Thorlabs Inc. (Newton, USA).

Embodiments described herein provide adjustment (automatic or otherwise)of the imaging beam's position relative to the processing beam, tocompensate for motion/misalignment. For example, such adjustment may becarried out automatically, to adjust the imaging beam for changes in thevelocity of the processing beam, and/or changes in the direction of theprocessing beam, and/or changes in the relative velocity between thebeam delivery system and the workpiece, and/or changes in the processbeam velocity and/or the velocity of the process beam's focus relativeto the workpiece. In some embodiments the imaging beam may be adjustedto compensate for flexing and dynamic misalignment in the processingbeam head and/or motion system. In some embodiments, the scanning deviceis used to adjust the imaging beam's position to compensate for flexingand dynamic misalignment of the head, the optics contained therein, andany motion system, such as, e.g., a robot or gantry motor that isutilized to effect the material modification process.

In some embodiments of the invention, an auxiliary measurement systemthat uses electronic, mechanical, optical, and/or capacitive techniquesis provided. It is used, for example, to determine the distance from thelaser head to the sample, and this information used as part of theinterferometry and/or feedback processing.

For example, in some embodiments, at least one imaging beam is used todetermine depth of the processing beam penetration in a material, i.e.,depth of the keyhole. In such an embodiment the at least one imagingbeam position is inside the keyhole. In various embodiments the at leastone imaging beam position inside the keyhole is used together with atleast one other measure to determine the depth of the keyhole. The atleast one other measure may be obtained, for example, using a mechanicalsensor in contact with a surface of the material being modified, anelectronic sensor, a capacitive sensor, or optically. For example, insome embodiments, at least two imaging beam positions include a positionoutside the keyhole and a position inside the keyhole.

Determination of keyhole depth according to embodiments described hereinmay be carried out simultaneously during the material modificationprocess. Such determination provides for dynamic control of the materialmodification process, thereby providing quality control, and improvingquality.

The imaging beams are used to measure, either on a one time basis, onceper workpiece basis, over a period of time, at regular or pre-setintervals, continuously, or substantially continuously, one or acombination of two or more keyhole features or parameters. The keyholefeatures, which are dynamic as they may change throughout the materialmodification process, include, but are not limited to, keyhole depth,location of maximum keyhole depth, average keyhole depth, keyholelocation, keyhole width, keyhole length, surface shape, subsurfaceshape, subsurface keyhole length, subsurface keyhole width, wall slope,sidewall angle, keyhole collapse, keyhole stability, dynamics of liquidregion of the PCR, location of interface between liquid and solidregion, other physical parameters of the PCR.

Embodiments described herein allow dynamic and simultaneous, orsubstantially simultaneous, tracking of the material modificationprocess, to achieve greater control of the process and higher qualityresults. In some embodiments, such tracking is carried out in respect ofthe processing beam, and accordingly at least one imaging beam issubstantially aligned with the processing beam. In other embodiments,such tracking is carried out with respect to the PCR, and in particular,in respect of one or more dynamic keyhole features such as thosedescribed above. Accordingly, in such embodiments, the imaging beam isaligned so as to determine the one or more dynamic keyhole feature(s).Such alignment of the imaging beam may not be with processing beam;rather, in such embodiments the imaging beam may track the PCR (i.e., aPCR feature) optionally by a selected distance or with an offsetdetermined so as to optimize the tracking.

The embodiments discussed in the description of FIG. 1 may beimplemented using the arrangement of FIG. 2.

FIG. 3 is a schematic diagram of an example of another apparatus thatimplements the coherent imaging in a material welding process. Amaterial processing laser 17 produces a laser output that is carried viaa fiber optic cable 26 and connects at fiber optic connector 25 to alaser head 28 which outputs laser beam 16. A laser beam 16 issubstantially collimated by optic 24, reflected by dichroic mirror 45and then focused by optic 18 such that keyhole welding is achieved onthe sample 12, 14. In some embodiments, mirror 45 is a movable mirror.

An imaging system 21 produces imaging light that is carried via fiberoptic cable 27 and connected to fiber optic connector 23 to the laserhead 28 where the imaging light is used to form an imaging beam 20. Acollimating lens 22 substantially collimates the imaging beam anddirects the imaging beam 20 towards the mirror 43. Adjustment of therelative positions and angles of 28, 27, 23, and 22 may be used tocontrol the position and focal plane of the imaging beam. A movablemirror is shown at 43. The mirror 43 is actuated by a motorized system(not shown) such that the imaging beam 20 can be directed to multiplelocations within the PCR 30, 32, 34 on the sample 12, 14. Instead of orin addition to a motorized mirror, an acousto-optic deflector,electro-optic deflector, or other device known to those of ordinaryskill in the art to statically or dynamically change the angle of a beamcould be used. In some cases, additional mirrors may be used to fold thebeam path to allow it to fit into smaller linear spaces.

The movable mirror 43 and the dichroic mirror 45 are used to allow thelaser beam and the imaging beam to be directed towards the sample andclosely aligned. Depending on the angles of the mirrors 43, 45, at agiven instant in time, the laser beam and imaging beam may be parallel,or at some angle relative to each other.

The imaging beam 20 is used to measure, over a period of time, one or acombination of two or more of the length, width, depth, surface shape,sub-surface shape, sidewall slope, collapse, instability, and/or otherphysical parameters of the PCR 30, 32, 34.

The embodiments discussed in the description of FIG. 1 may beimplemented using the arrangement of FIG. 3.

Features denoted by reference characters 41, 42, 46 appearing in FIG. 3will be detailed below in the discussion of working examples.

In some embodiments of the invention, a sacrificial covering glass isprovided to protect the rest of the optics from emissions from thematerial modification process. The use of cover glass in laser materialsprocessing is widely known and used by those of ordinary skill in theart.

In some embodiments of the invention, a cross jet of gas from one or aplurality of gas-fed pressurized orifices (known by some as an airknife) is employed to protect the optics from emissions from thematerial modification process by blowing them away. The use of a crossjet is widely known and used by those of ordinary skill in the art.

In some embodiments of the invention, a cover gas applied by one or morenozzles located above and/or below the workpiece and is employed toproduce/prevent specific chemical effects (e.g., reduce oxidation) onthe material being modified. The use of a cover gas is widely known andused by those of ordinary skill in the art.

The use of cover and cross jet gases may also provide the benefit ofsuppressing the amount of plasma and/or debris inside the beam path,which changes how the energy from the high power processing beam isabsorbed in the sample.

Coherent imaging is particularly well suited to observing andcontrolling high energy material modification processes for severalreasons. It is resistant to sensing process light, it is virtuallyimmune to blackbody radiation, it has high sensitivity, high dynamicrange, high speed, and is easy to integrate into existing opticalsystems.

Use of Auxiliary Measurement System to Influence the Feedback Output

In some embodiments, the beam delivery system may be modular to allowmultiple configurations of optics, gas, material processing beams, andinstrumentation to be used. Such instrumentation includes auxiliarymeasurement tools that can be used to influence the feedback and/orimaging output. Several examples follow:

-   -   1. The beam delivery system may use an auxiliary capacitive        sensor to determine its separation from the sample.    -   2. The beam delivery system may include additional ports to add        auxiliary instrumentation such as a co-axial camera and/or a        laser triangulation system.    -   3. Structured light topology is an extension of laser        triangulation and can also be used in conjunction with the beam        delivery system for auxiliary measurements of the workpiece.    -   4. The feedback controller may also receive information from        auxiliary equipment mentioned in the previous points and        incorporate this information into the feedback output it gives        to the process control system.    -   5. The image processor may also receive information from        auxiliary equipment mentioned in the previous points and        incorporate this information into the processing algorithms that        it uses. For example, if the working separation distance between        the delivery optics and the sample is detected to have increased        by 1 mm, then the image processor can subtract 1 mm from the        measurements of weld depth that it produces.        Compensation for Gas Pressure in the Coherent Imaging Beam Path

In some embodiments, optical path length compensation for gas pressurein the coherent imaging beam path is performed. The process controllerand/or image processor may receive inputs from one or more pressuresensors that read the ambient pressure inside locations in the beamdelivery system. Gas pressure changes the optical path length registeredby the coherent imaging system. In some embodiments, these measurementsare used to digitally (i.e., in processing the signal electrically or insoftware) or physically (e.g., by modifying the reference arm length)compensate for optical path length changes due to high pressure gas.

The amount of correction required is obtainable by fixing the locationof the process head relative to a solid object and graduallypressurizing the head while tracking the apparent location of thesurface. Thus a mapping of path length error as a function of headpressure can be obtained. Similarly, if the gas composition has asignificant effect on the path length, this too can be simulated andmeasured in the same way. After programming this mapping into theprocess controller/image processor, it can then apply the necessarycorrection based on the measured pressure inputs.

Compensating for Optical Path Length Changes Resulting from Scanning theImaging/Processing Beams

In some embodiments, compensation for optical path length changesresulting from scanning the imaging/processing beams is performed. Thiscan be done digitally and/or physically by adjusting an optical pathlength in the interferometer. This can be done, for example, by mirrorsor by adding glass.

In some embodiments of the invention, the optical path length imagingbeam on the sample is physically modified as a consequence of directingit to a different location on the sample, even if the sample heightitself is not changing. This results in a distortion of themorphological data that the imaging system returns. While this can becorrected digitally in some cases, it may also be beneficial to correctfor it physically, especially if the distortion is large. Alternatively,it may also be advantageous to intentionally distort the imaging fieldin order for it to conform to a material geometry that is not flat. Thedecision whether or not to take any of these approaches is applicationspecific.

The first step to a correction is to measure the distortion. This isaccomplished by moving the imaging beam to several locations on anobject that is known to be flat, measuring the apparent height of theobject in the coherent imaging system and subtracting the desiredprofile from these results. This data yields the error between theuncorrected and desired profiles.

The next step is to apply the correction. Digitally, this can beaccomplished by addition of the known error at given positions to thefuture measurements at or around those positions. Error maps can beinterpolated beforehand or in real time. Physically, distortioncorrection can be accomplished by manual or automated addition ofoptical materials to the reference path. This includes modifying thereference arm delay line length. In production, this can be co-ordinatedwith the scanning device. For example some scanning devices (such as aCambridge Technology FlexScan-3D™, Cambridge Technology, Inc., Bedford,USA) utilize a Z-axis actuator to control the focal distance of thelaser beam. The control signal for this actuator can be utilized as acommand signal (e.g., via digital/analog scaling and subtraction) forthe reference arm of the coherent imaging system.

Acoustic Excitation to Detect Liquid vs. Solid Phases

In some embodiments, acoustic excitation is employed to detect liquidvs. solid phases. This might be used, for example, to provide anindication of the viscosity of the melt. Coherent imaging can sense thefrequency and amplitude of a vibrating melt pool by monitoring itsaxial/transverse position and/or its reflectivity (which indicates,amongst other things, the radius of curvature of its surface). Ifvibration of a melt pool can be excited by optical (e.g., laser) ormechanical (e.g., an acoustic transducer) means, then coherent imagingis able to sense the presence of these vibrations, their frequencyand/or their amplitude. This can be used to measure the viscosity of themelt pool in a certain region and to distinguish between liquid andsolid metal.

For example, immediately after a spot weld occurs, a molten pool ofmetal is left oscillating. Coherent imaging data can determine theperiod and/or phase of the oscillation of the melt pool. This period ischaracteristic of and can therefore be used to determine at least one ofthe melt viscosity, material type, material state, and geometry. Bysensing the period, (in this case) one can infer that the melt poolwidth is decreasing over time.

FIG. 5 shows an example of a coherent image of a laser spot weld. Thekeyhole is initially opened and increases in depth until the laser pulse(550 us duration, 1 kW intensity) is terminated at about a quarter ofthe way into the image. The keyhole remains open for a few tens ofmicroseconds until it is filled in by liquid metal. The liquid metaloscillates which can be seen by the repetitive enhancement and reductionof the signal. Notably, the period of the oscillation is steadilyreduced as the size of the oscillating weld pool shrinks due to theperiphery fusing.

Detecting Spiking in Weld Depth

In some embodiments, spiking in weld depth is detected, and used as anindicator of the presence of porosity in the weld. Porosity indicatesstrength/longevity and sometimes even corrosion resistance of the weld.It is known to those of ordinary skill in the art that a weldingphenomenon called “spiking” exists. Spiking is characterized by rapid,momentary enhancements in the weld penetration depth. In post-welddestructive testing, spiking appears as an inconsistent weld depth withone or many narrow depth enhancements. The underlying cause of spikingis dynamic instability and chaotic behaviour of the materials in andaround the keyhole. Coherent imaging techniques are fast enough and havehigh enough spatial resolution to detect the rapid and momentaryenhancements in depth that are the spiking phenomenon. Consequently,coherent imaging techniques can sense keyhole instability.

Also associated with keyhole instability are weld defects such asporosity and bad mixing of dissimilar materials. There is a correlationbetween the appearance of spiking dynamic behaviour as sensed bycoherent imaging techniques and the presence of porosity in a weld.Therefore, by observing the keyhole dynamics of welding, coherentimaging can detect the presence of porosity in the weld.

Furthermore, by sensing the phase of keyhole oscillations with coherentimaging, the process controller can be made to synchronously orasynchronously drive the keyhole oscillations by modulating the power ofthe high power laser, or through the application of another energysource such as from an ultrasonic transducer.

Zinc is widely used to inhibit corrosion on a variety of metal products.The low vaporization temperature of zinc presents significant challengesto welding of materials that contain zinc (e.g., galvanized steel). Theexplosive vaporization of zinc can eject material from the phase changeregion and is difficult to predict. This can result in underfill andunwanted weld depth enhancements. Coherent imaging can sense theoccurrence of a gas phase explosion or similar transient and generate anannunciation that can be used to warn the user or a process controllerthat a gas phase explosion or similar transient may have compromised thequality of a weld. In coherent imaging, this condition looks like asudden enhancement of the weld depth over the course of a time period,for example ˜5 ms. Coherent imaging can also sense keyhole instabilityfrom small transients before a large transient and generate a warning ofrisk of a future large transient or large gas phase explosion. This maybe thought of in the way that small seismic events may herald the comingof a large one in the future. In this case, corrective action can betaken to prevent a large zinc explosion by, for example, reducing thepower of the processing beam and/or increasing the gap between two partsto be welded by e.g., tens of micrometers. While zinc is referred to inthis paragraph, other materials may have the same effect.

Referring now to FIG. 10, in the top view, a coherent image of a lapwelding process involving galvanized steel and a 0.015 inch thickstainless steel shim is shown. The material is fed at 60 mm/s and imagedat 230 kHz. The image processor is configured to track the bottom of thekeyhole and has overlaid a lighter area to enhance the contrast of theimage for easier viewing and interpretation of welding depth. Thetracking algorithm locked on to signals 10 standard deviations above thenoise floor and used a transverse correlation length of 0.05 mmHorizontal lines are overlaid showing the thickness of the top sheet.This same underlying tracking information can also be used for numericalinterpretation of the keyhole depth and instability. Severe spiking andinstability are clearly shown by the large depth swings (amplitudes >0.6mm are evident over time scales of 5-10 ms and lengths of ˜5 mm). Thiswould likely be a defective weld.

In the bottom view, substantially less instability in a similarcontinuous wave (CW) laser weld is evident. However, there are a few“spikes” visible at 1 mm and just before 2, 3, 4.5, and 8.5 mm. It ismore likely that this weld is a good weld than in the top view.

Use of Coherent Imaging in Conjunction with a Mechanically Actuated Lensto Keep the Focus of the Processing Beam a Selected Distance from theMaterial Surface

In some embodiments, one or more of the coherent imaging systems ormethods described herein is used in conjunction with a mechanicallyactuated objective lens to keep the focus of the processing beam acertain distance from the material surface. This approach may beparticularly useful in laser cutting embodiments as opposed to weldingembodiments. In one example, the focus of the processing beam ismechanically actuated over distances between 0 and 5000 mm along theaxis of the beam. This capability is demonstrated without coherentimaging in products such as the Laser Mechanisms FiberCut RA™ (LaserMechanisms, Inc., Michigan, USA). This may be desirable to correct fornon-ideal material geometry (thickness, distortion, etc.) and/ornon-ideal motion of the beam delivery system (e.g., a robot that isunable to smoothly move over a plate of metal). If a coherent imagingsystem were to be added to such a scheme, it could be used instead of,or in addition to, a capacitive height sensor. In this case, thedistance of actuation is likely to roughly correspond to a change in theworking path length between the coherent imaging system and thematerial. It may therefore be desirable to change the reference pathlength of the coherent imaging system in a way that is correlated withthe actuation of the processing beam's focus, in order to reduce therelative path length between the sample and reference arms. In mostcoherent imaging systems, this serves the function of keeping thematerial surface visible inside of the field of view.

One example of how to produce this correlated reference path lengthactuation is to digitally synchronize a reference mirror actuator (whichmay be located some distance away from the beam delivery system itself)to the processing beam focus actuator. An example actuator for thereference arm is an Aerotech PRO115™ linear translation stage (Aerotech,Inc., Pittsburgh, USA). Less expensive and mechanically precise linearactuators may also be used. If the reference path attached to theactuator has multiple passes (i.e., the unidirectional beam path crossesthe expanse that is modified by the actuator), then the actuator may beprogrammed to move a corresponding fraction of the distance moved by theprocessing beam focal adjuster.

Another example of how to produce the correlated reference pathactuation is to use a surface attached to the processing beam focalactuator itself as the reference surface. In this example, somecomponent of the reference path exists inside the beam delivery systemand is reflected by a retroreflector or a mirror that is mounted on theprocessing beam focal actuator. In this way, as the focal position ischanged by the actuator, the reference path length is automaticallycompensated. It may be desirable to mount a lens above the referencesurface in the beam delivery head that converges the beam on saidreference surface in order to make the coherent imaging signal lesssensitive to mechanical imperfections in the focal actuator. Theconfiguration described in this example also provides the benefit ofbeing used as an autofocus system. The focal actuator is programmed viathe image processor and feedback controller to keep the material surfaceat user-defined distance from the zero-delay point in the coherentimaging system.

In order to reduce dispersion mismatch between the reference and samplepaths, it is desirable to use approximately equal amounts of eachoptical material in both paths (this includes air/vacuum). It may bechallenging and/or impractical to produce a sufficiently long air pathfor the reference entirely within the beam delivery system. It may alsobe very problematic if the process lens, i.e., lens A in FIG. 6 or lens109 in FIG. 7, was exchanged for one with a different focal length.Therefore, in the example mentioned above, the air component of thereference arm is broken into two sections. The second section is the onethat reflects off the processing beam focal actuator as described above.The first section of the reference path is produced at another locationinside the coherent imaging system by coupling out of the optical fiberand into the air and then back into fiber before transmitting thereference light to the second section in the beam delivery system. Thisfirst part of the path can be adjusted to configure the amount ofreference signal power desired, the dispersion characteristics of thereference path, and provide fine adjustment of the zero-delay pointrelative to the material and/or the focus of the high power processingbeam.

In the case that the coherent imaging system does not share the samefocal objective as the processing beam, then the focal position of thecoherent imaging system may also be actuated axially to keep the focusof the coherent imaging system near the surface of the material beingmodified.

An example of such a system is depicted in the embodiment of FIG. 6.FIG. 6 shows a reference arm that has an adjustable delay line, and alsothat has a delay tied to the position of the process lens carrier, anadjustable focal position, and that has capacitive height sensing. Moregenerally, one or more of these features may be implemented. In someembodiments, one or more of these features is implemented in combinationwith any of the methods/systems described herein.

In FIG. 6:

fiber lengths for sample and reference arms are approximately matched;

not shown is the presence of dispersion compensating media located inthe adjustable delay line and/or the nozzle via path x2;y1+y2+y3+x2=x1+x2+x3 (approximately);

as focal position is adjusted dynamically according to the position ofthe process lens carrier (i.e., location of lens A is shiftedvertically), the adjustments are automatically matched by the end mirrorlocated on the same mechanics as lens A;

if the focal length of lens A is changed (e.g., lens A is replaced by adifferent one), then the lengths of y1, y2, y3 can be changedaccordingly, and in some embodiments programmatically;

in some embodiments y1, y2, y3 are adjustable to compensate for changesin gas pressure or other distortions;

distance x2 which forms part of the reference arm tracks movement of theadjustable focal position provided by the process lens and end mirrorcarrier; and

a capacitive height sensing mechanism is provided.

Other Imaging Light Sources

The coherent imaging system may be of the time-domain, spectral domain(i.e., spatially multiplexed spectral measurements) or swept-source(i.e., temporally multiplexed spectral measurements) types. In the firsttwo cases, a superluminescent diode and/or broadband laser (e.g., modelocked Ti:Sapphire, mode locked fiber laser) is an example of anacceptable light sources for coherent imaging. A microelectromechanicalsystem tunable vertical cavity surface emitting laser, and a MEMS(micro-electromechanical system)-tunable VCSEL (vertical cavity surfaceemitting laser) (see for example Benjamin Potsaid, et al. “MEMS tunableVCSEL light source for ultrahigh speed 60 kHz-1 MHz axial scan rate andlong range centimeter class OCT imaging”, Proc. SPIE 8213, OpticalCoherence Tomography and Coherence Domain Optical Methods in BiomedicineXVI, (Feb. 9, 2012); see also Thorlabs MEMS VCSEL Swept Source OCTSystem, Thorlabs Inc., Newton, USA). Light sources with very longinstantaneous coherence lengths allow for longer imaging ranges that maybe particularly beneficial for observing and controlling the materialmodification processes described herein and those described throughreference.

Swept-source and time-domain imaging approaches typically do not usedetectors that can substantially discriminate between differentfrequencies of light. This means that they are more vulnerable to beingoverloaded by the incoherent emissions of the process and/or by the highpower modification energy. The addition of blocking filters, eitherinside the fiber line (e.g., fiber Bragg gratings, etc.) or at thedetector, to isolate the imaging light from the unwanted signals may beemployed for the material processing applications described herein.Balanced detection is also substantially beneficial for rejecting theseunwanted signals.

Other Material Processing Beam Sources

While the majority of the examples presented here concernfiber-delivered lasers operating in the 1000 to 1100 nm wavelengthrange, this approach is agnostic to the wavelength of the high poweredmaterial modification beam. For example, the wavelength of themodification beam may be between 1 nm and 50 μm.

Digital Compensation for DC Signal Changes Arising from Reference PathActuation

In some embodiments of the invention, the reference arm mirrors may bemoved in order to adjust the optical path length of the reference arm.Due to precision limitations in the motion control and the sensitivityof energy coupling into single-mode optical fiber, the DC intensity ofthe reference signal may change with the position of the reference armdelay line. In order to reduce the appearance of fixed pattern noise inthe image when the DC power level changes, the background subtractionarray may be scaled to match the current DC power level. The amount ofscaling can be determined a priori/offline by mapping the DC power levelfor some set of reference arm positions, or in real time by minimizingthe DC power signal (after the signal has been converted from aninterferogram) through changing the scaling.

Combining the Processing and Imaging Beams after their Respective FocalObjectives

In some embodiments, the processing beam and the imaging beam(s) arecombined after their respective focal objectives. Such a system has abeam combining device located distally (i.e., towards the sample) afterthe focusing devices for the processing and imaging beams. The combinermay be used as a cover glass to segregate other optics from the processgas.

In the case of a high power laser processing beam, the combiner may be,for example, a multilayer dielectrically coated optic that transmits thehigh power processing beam and reflects the imaging beam. Reflecting thehigh power processing beam and transmitting the imaging beam is alsopossible.

The combiner may also be used to sample the intensity of the high powerprocessing beam by directing it to an optical power meter subsystem ofthe beam delivery system.

Independent lenses allow for scanning both the high power processingbeam and the imaging beam and maintaining the alignment of the imagingsystem relative to the process. Otherwise, chromatic aberrationsinherent in the focal lenses might cause the imaging and processingbeams to walk off of each other, particularly at larger scan angles.

An example of such a system is depicted in FIG. 7. The elements of FIG.7 include the following:

-   -   101. Sample arm collimator for coherent imaging system    -   102. Fiber delivery of coherent imaging system    -   103. Coherent imaging unit    -   121. Feedback controller    -   122. Process controller    -   106. Coherent imaging tuning and/or position tuning mirror    -   104. Galvanometer scanner (1, 2 or 3 axis)    -   105. Scanning lens for coherent imaging system    -   107. Combining device for processing and imaging beams    -   108. Power sensor for processing beam    -   109. Focusing device for processing beam    -   110. Cover glass    -   111. Cross jet/air knife    -   112. Turning mirror for processing beam    -   113. Turning mirror for camera port    -   114. Filter for CCD (charge coupled device) camera    -   115. CCD camera lens    -   116. C-mount    -   117. CCD/CMOS camera    -   118. High power collimator    -   119. High power connector    -   120. High power delivery fiber    -   123. Camera port    -   124. Coherent imaging port        Not shown is a power control input to the laser, cover gas        nozzle, or wire feed/arc tip, any of which may be included in        some embodiments.

Mirror 112 in FIG. 7 could optionally be a pair of mirrors and could bemotorized to allow for “remote” welding. Element 108 could be a powerdetector or simply a power absorber. Furthermore, it is noted thateither scanning mirror pair, or the adjustable mirror of any of theembodiments described could be replaced with an acousto-optic orelectro-optic deflector.

Note that the approach of FIG. 7 has the benefit of not requiring anF-theta lens to be used with the processing beam.

Hybrid Laser Arc Welding

All of the techniques described herein may be applied to observe hybridlaser arc welding or any other material modification process that has avapor channel/capillary, such as, for example, material liquefaction.

Multiplexing the Reference Arm

In some embodiments of the invention, multiple imaging beams are focusedon different areas of the sample. It is not required to have a separatelight source and detection scheme for each of these points. Instead,they can be multiplexed onto the same detector (as demonstrated in FIG.4B). To accomplish this, one may split the sample arm into one or morepaths/channels (e.g., by way of an evanescent mode coupler, or multipleinternal reflections within an optic) with different optical pathlengths. In some cases, the difference in these lengths would be greaterthan the depth field of view of the coherent imaging system, which maybe preferable depending on the application and the number of channels,but not required. The reference arm can be configured to generate one ormore reference reflections to correspond to each of these differentpaths. The actual splitting of the reference arm itself can beaccomplished in many ways that would be known to those of ordinary skillin the art (e.g., with semi reflective mirrors, or by splitting thereference into multiple paths each with their own configurable delay),but one must carefully plan the different path lengths to match theexpected sample paths during use. Each reference arm path would beconfigured to add enough additional delay to each channel such thattheir signals would not overlap. In some embodiments, channels withlower SNRs (e.g., signals from within the keyhole) would be configuredto be closer to the zero delay point than those with higher expectedSNRs. The result is that each sample image channel would appear at adifferent location in the processed image and can therefore besimultaneously observed without the need for an entirely separatecoherent imaging system.

Another way to effectively multiplex the reference arm is to utilize thesecondary coherence subpeaks that are exhibited by some low-coherencelight sources (due to residual Fabry-Perot modulation) such as thosesold commercially by Superlum (Carrigtwohill, Ireland). These subpeakscreate an interference signal typically several millimeters away fromthe main lobe of the coherence function. This separation can beeffectively used as if there was a separate reference surface severalmillimeters away from the primary one.

In another embodiment of the invention, a birefringent optic (such as acalcite prism) is used to multiplex the imaging beam and spatiallyseparate it onto different parts of the sample. The polarization of theimaging beam into the birefringent optic can then be adjusted to balancethe intensity between the two sample paths. Similarly, otherbirefringent optics can be used in the reference arm to createpolarization-specific optical delays for interference comparison and/orto address/access the two polarization channels.

Detection of Surface Composition and/or Quality

Surface composition and/or quality (including, e.g., an indication ofoxidation condition, and roughness) is an important factor in theacceptance of a particular material modification process. As the imaginglight from a coherent imaging system interacts with the materialsurface, it can provide a measurement of surface roughness and/oroxidation. This information can be extracted from relative intensitymeasurements (i.e., comparing signal intensities from differentlocations and/or different materials) and spectral measurements (i.e.monitoring signal attenuation in a particular region of the imagingspectrum). All of these signals can be processed through the imagingprocessor and be used to generate various annunciations and processfeedback as desired.

Location of Reflecting Interface by Intensity Peak Pixel Identification;Centroid Fitting; Phase and Fringe Evaluation

In some embodiments, when processing coherent images, the location ofreflecting (and/or scattering) interfaces is determined by locating thedepth element with the peak signal intensity (peak pixelidentification). In some embodiments, the location of reflectinginterfaces is achieved by fitting a quasi-Gaussian curve and extractingits centroid. In some embodiments, the location of reflecting interfacesis achieved by measuring the phase and location of the fringes in theinterferogram. In some embodiments, combinations of two or all three ofthese methods are used to achieve the location of reflecting surfaces.In some embodiments, certain signals are selected and others aredisregarded (e.g., in the case of background noise or secondaryreflections, to choose a region of interest, or the like).

Interferometry Examples

Interferometery is used throughout the embodiments described herein togenerate the measurements discussed. In some embodiments, theinterferometer comprises: a combiner; a reference arm, a first componentof the imaging light being applied to an input of the reference armresulting in an output signal of the reference arm, the reference armhaving another optical path length; and a sample arm, a second componentof the imaging light being applied to the sample arm resulting in anoutput signal of the sample arm, at least a component of the outputsignal of the sample arm including reflections of the component of theimaging light from a sample location, the sample arm having at least oneoptical path length; wherein the combiner combines the output signal ofthe reference arm and the output signal of the sample arm to produce acombined signal as an interferometry output; the apparatus furthercomprising a signal detector configured to produce a first interferogramfrom the interferometry output.

In some embodiments, the apparatus comprises at least one of: multiplesample arms, a respective interferogram being generated for each samplearm, reference arm combination; multiple reference arms, a respectiveinterferogram being generated for each sample arm, reference armcombination; and multiple reference arms and multiple sample arms, arespective interferogram being generated for each sample arm, referencearm combination.

In some embodiments, the interferometer comprises: at least one splitterand/or optical circulator; and at least one sample arm after thesplitter and/or optical circulator, the imaging signal being applied tothe sample arm resulting in an output signal of the sample arm, at leasta component of the output signal of the sample arm including reflectionsof the component of the imaging signal from at least two locations inthe sample arm and/or the material being processed, the sample armhaving at least one optical path length and another optical path length;wherein the splitter and/or optical circulator receives the outputsignal from the sample arm and directs it towards a detector; theapparatus further comprising a signal detector configured to produce aninterferogram from the interferometry output.

FIG. 8 is a block diagram of a first detailed implementation. In thisembodiment, modification beam (ML) 200 and imaging beam (SLD) 204sources are shown. In this embodiment, the two light paths are combinedby a dichroic or other combining optic (DM) 206 after independent focalobjectives 208, 210. In this embodiment, the interferometer can be builtin single or, in other embodiments, in multi-mode optical fibre.Detection is accomplished by means of a high speed spectral detector(HSS) 212. While the embodiment shown uses a 50:50 power splitting ratio214 between sample arm 216 and reference arm 218, in other embodimentsother splitting ratios in the interferometer are possible and may dependon the availability of optical power and/or the need for detectionsensitivity. In some embodiments, other interferometer configurations,e.g., Mach-Zehnder, Sagnac, common path, etc. may be used. In thisembodiment, DM 206 is shown to reflect the imaging light and transmitthe modification light, however, the reverse can alternatively beimplemented. In some embodiments, combination of the beams viapolarization-sensitive or neutral reflection optics may be implemented.A person of ordinary skill in the art will understand that detection,processing, and feedback electronics are omitted from the embodimentshown in this figure and such processing steps may be performed withinthe feedback controller. Feedback controller 214 receives the output ofthe HSS 212 and controls the modification laser 206 and/or one or moreother aspects of the material modification process.

FIG. 9 is a block diagram of a second detailed implementation. In thisembodiment, a high power broadband source is created by coupling short,dispersion-optimized pulses output by broadband source 300 into a lengthof single mode optical fiber 310. This results in an expansion ofspectral bandwidth, in some embodiments, on the order of a factor of 6,though in other embodiments, more or less broadening is possible. Theembodiment shown features a Ti:AlO3 laser source 301 that operates inthe region of 650 to 1100 nm. In other embodiments, spectral ranges from300 to 15000 nm from other optical imaging sources may be used. In thisembodiment, a Glan-Taylor polarizer (GTP) 302, Faraday optical isolator(ISO) 304, half-lambda waveplate polarization control 305, and Forkprism dispersion compensation 306 are shown. In other embodiments, otherbroadband sources (such as superluminescent diodes, other lasers, and/orother broadening methods) may be substituted for the broadened Ti:AlO3laser source.

In this embodiment, the modification laser (ML) 320 passes throughcollimator 351 and the imaging beam passes through sample arm collimator353 after which the modification laser beam and the imaging beam arecombined by an optic component (DM) 312 before they are focused by acommon focal objective 314.

In such embodiments, the lens may be achromatic, aspheric, and/orconical (e.g., axicon). This beam combination may be focused through anoptional nozzle 316 that can be used to apply assisting fluids (e.g.,compressed gas, water spray) to the modification process. The nozzlespray may also be independent from the optical beam; i.e., the two aredelivered to the sample from different points. The Michelsoninterferometer includes a 50:50 splitter 322 (though in otherembodiments, other splitting ratios may be used), reference armcollimator 355, and reference mirror 326. Also shown are polarizationcontrollers 324, 325, 330. The spectral detection in this embodimentinvolves a fiber-coupled reflective grating spectrometer 318. In someembodiments, an additional mirror in front of the lens (ASL) 321 canallow the beam to approach and leave the reflective grating 318 as closeto the Littrow configuration as possible, improving diffractionefficiency. In some embodiments, a transmission grating and/ormulti-grating, and/or Fabry-Perot spectrometer may be used. A siliconline camera 330 produces an interferogram that is passed to imageprocessing electronics 332, the output of which is passed to feedbackcontroller 334. Feedback controller 334 produces a feedback 336 tocontrol the modification laser 320 or some other aspect of themodification process.

It will be understood that the methods and apparatus described hereinmay also be used in applications in “remote” welding heads (such as, forexample, the Scanlab AG IntelliWELD™ head, Scanlab AG, Germany) wherethe imaging system is introduced into the camera port of the head. Inthis case, any splitting or scanning of the imaging beam is still done,but since the processing and imaging beams share the scanning mirrorsinside the remote head, the imaging beam is always positioned relativeto the processing beam. In other words, the IntelliWELD head's mirrorsprovide a “common mode” deflection of the imaging and welding beams andan additional scanner/splitter (such as the one depicted in FIG. 3,element 43) produces offsets between the imaging and processing beamsfor the purposes of imaging various locations in the vicinity of thePCR.

In another embodiment, the remote head utilizes scanning mirrors afterthe focusing objective (i.e., “post-objective scanning”), therebyminimizing chromatic aberrations and/or chromatic walk-off of theprocessing and imaging beams that might be present when using an F-thetascan lens.

In some embodiments, an active deflection element (such as the one shownin FIG. 3, element 43) is programmed to deflect the imaging beam by anangle that is related to the current velocity of the processing beamrelative to the workpiece. This is because many of the features ofinterest of the PCR lag behind the processing beam by an amount that isrelated to the processing velocity. Such dynamic lag control may beparticularly acute when the process is changing velocity, magnitude,and/or direction, such as in a curvilinear weld. The amount of lag maybe preprogrammed with the process, automatically looked up from a storedtable, or dynamically calculated, e.g., from a fitting function that ispreprogrammed at the time of manufacturing or adjusted in the field.

In some embodiments, the apparatus may be programmed to automaticallysearch via scanning for a specific PCR feature, and recognize whenproper alignment is achieved. For example, proper alignment with thebottom of the vapor channel is achieved when a sufficient/optimalfrequency and strength of a reflected signal is detected from asubsurface depth. Through a process of, for example, initial offsetestimation, dithering, and successive iterative adjustment, the optimalPCR lag for a linear or curvilinear weld path may be found. Suchautomatic alignment lends itself to considerable ease of utilizing theembodiments described herein.

Active deflection adjustment and dithering is also useful formaintaining alignment with the PCR if changes to the optics (e.g.,arising from vibration, wear, temperature shifts, thermal lensing, orrebuilding of the optics).

Other examples can be found in U.S. patent application Ser. No.13/245,334, and International Patent Application No. PCT/CA2011/050599,both hereby incorporated by reference in their entirety.

Working Example—First Experiment

A first experiment will be described in the context of the exampleapparatus of FIG. 3. For the experiment, the laser 17 was a 1070 nm, 1kW IPG fiber laser, and the laser head 28 was a Laser MechanismsAccuFiber™ head (Laser Mechanisms, Inc., Michigan, USA). The imagingsystem 21 was connected to the laser head 28, and produced imaging lighthaving a 840 nm central wavelength (and a bandwidth of approximately 20nm measured at full-width, half-maximum) which was reflected off amovable mirror 43. A dichroic mirror 45 was used to allow the laser beamand the imaging beam to be directed towards the sample while beingclosely aligned. A sample of mild steel plate was moved at rates ofapproximately 20 mm/s in a linear direction 46 while continuous wavekeyhole bead on plate welding was performed. Weld depths ofapproximately 2 mm were achieved. Trials were performed with the imagingbeam 20 directed at various positions (e.g., 20 a-20 e of FIG. 1)relative to the laser beam 16 such that multiple measurements within thekeyhole and PCR 30, 32, 34 were obtained.

FIGS. 4A-4E are coherent images of keyhole laser welding. This series ofimages demonstrates the effect of leading, aligning, or trailing theprocessing beam with the imaging beam. When the imaging beam leads theprocessing beam, a poor view of the keyhole depth is achieved in thisexample, in part because the bottom of the keyhole is poorly illuminatedby the imaging beam and in part because much of the imaging light isreflected away and lost by the front wall of the keyhole, neverreturning to the imaging system. When the imaging beam is aligned withthe processing beam, image quality and signal-to-noise ratio (SNR)improves, but there is significant vertical scatter as this locationstill includes a great deal of the front keyhole wall, which can bequite unstable in the axial direction. Significant improvements to SNRand vertical stability are seen by trailing the processing beam by afraction of its width. When the processing beam is trailed by an evenlarger amount, both the top surface and the bottom of the keyhole can bemeasured. This important capability allows for dynamic determination ofthe top surface reference points (TSRP) and is useful in cases wherethere is distortion of the material and/or of the motion system. Theseimages are described in more detail below.

In FIGS. 4A-4E, the x-axis represents time, readings were taken at arate of 100 kHz (time bins on the x-axis are separated by 10 us), andbecause the sample was in motion at 60 mm/s, this axis also correspondsto distance along the sample during the weld. The y-axis representsheight above the zero delay point of the interferometer. The zero delaypoint is placed below the surface of the material such that depth isincreasing as the signal approaches the zero delay point at the top ofthe image (labelled 0), i.e., increasing depth is in the upwarddirection. The black pixels in the image correspond to the depth of thereflection of the imaging beam off the sample surface, darker pixelscorrespond to a brighter reflection. The black pixels appear to bescattered in part because the keyhole is constantly changing. Thediameter of the imaging beam is approximately 70 μm and the diameter ofthe processing beam is approximately 200 μm.

FIG. 4A depicts image data where the imaging beam leads the processingbeam by 25 μm and roughly corresponds to a position between FIGS. 1, 20a and 20 c. The sample begins to move and reaches a steady speed beforethe 0 time mark (left edge of the image) Imaging begins some 100 usbefore the laser turns on and is used to establish the TSRP of 3624 μm.After the laser turns on, a keyhole is rapidly created (in the first1000 time bins, corresponding to 10 ms) as shown by the black pixels.The keyhole at this position then remains open with measurements ofbetween 1200 μm and 1800 μm. This gives a depth reading of approximately1.8 and 2.4 mm, getting deeper as the weld progresses. The laser turnsoff at 67500 time bins and the weld terminates and the motion stopsafter approx. 76000 time bins covering a distance of 45.6 mm After thistime, the imaging system continues to run virgin surface only. In FIG.4A, the front wall of the keyhole very close to the leading edge isbeing examined. It can be seen that there is some scatter in the datarepresenting the dynamic variations of the keyhole front wall positionand slope.

FIG. 4B depicts image data where the imaging beam aligned with theprocessing beam and roughly corresponds to position 20 b of FIG. 1. Thisimage has a much higher signal to noise ratio (i.e., the signal isbrighter), making it easier for a viewer or image processor algorithm tolocate the keyhole bottom. As a result, one can more clearly see theinitial formation the keyhole as the black pixels move deeper in thefirst 5000 time bins of the image. The keyhole depth remained fairlyconstant between 2 and 2.2 mm below the TSRP (located at 2890 μm in thisimage), getting slightly shallower towards the end of the weld. Thisimage also demonstrates a feature of some embodiments of the invention.Here, in addition to the main imaging beam measuring the bottom of thekeyhole, a secondary beam path is created by a multiple reflectioninside the dichroic (beam combining) mirror. This secondary beam isdelayed relative to the first (i.e., appears at a different depth in theimage) and impinges on the sample at a separate transverse locationsubstantially outside of the PCR on the virgin metal. This secondarypath also creates an interference signal that shows up in the image atthe indicated depth 650 μm. Because this transverse location issubstantially outside of the PCR, the capability to track the virginsurface simultaneously with imaging the bottom of the keyhole ismaintained. This allows for dynamic measurement/calculation of the TSRPand/or height.

FIG. 4C depicts image data where the imaging beam trails the processingbeam by 50 μm and roughly corresponds to position 20 c of FIG. 1. TheTSRP was measured to be at 3120 μm in this image. Here the keyholeappears even brighter than in FIG. 4B with a depth of between 2 mm and2.2 mm There is less vertical scatter in the image showing that thekeyhole has more stability at this 20 c location.

FIG. 4D depicts image data where the imaging beam trails the processingbeam by, e.g, 65 μm, and roughly corresponds to position 20 d of FIG. 1.After initiation the keyhole depth is approximately 2 mm. The TSRP wasmeasured to be at 2700 μm at the beginning of the image, and 3400 μm atthe end of the weld. As a result, the weld depth appears in this imageto change by some 700 μm across the weld, despite not actually changing.This image illustrates the challenge of applying coherent imagingtechniques accurately without a well-defined TSRP. One way of obtaininga TSRP for all points during the image is to assume that the change inTSRP is linear with time and so can be interpolated from the beginningand end TSRPs. In many cases, this is sufficient. Note that this imagingbeam is trailing the processing beam sufficiently to illuminate thekeyhole bottom and the top surface of the rear of the weldsimultaneously. If one draws an imaginary line between time 0, depth2700 and time 67000 and depth 3400, several (approximately 100) pointsof enhanced signal can be seen. These are reflections from just behindthe keyhole in the region that will form the solidified surface of theweld. Since the top of the weld is at about the same height as thevirgin surface of the material, these signals can be used for additionalTSRP measurements. This feature of the embodiments is clarified furtherin the next figure.

FIG. 4E depicts image data where the imaging beam trails the processingbeam by 75 μm and roughly corresponds to a position between beams 20 dand 20 e of FIG. 1, such that both the surface of the weld and thebottom of the keyhole are illuminated at some time. The TSRP wasmeasured to be at 2050 μm at the beginning of the image. Here the blackpixels are gathered in two bands at different depths. The deeper (closerto 0 in the image) line corresponds to the bottom of the keyhole at the20 c/20 d locations. The shallower upper line (in this example the TSRP)corresponds to reflections off the liquid phase of the PCR as would beseen at location 20 e. The signal intensity that returns from eitherregion depends on how much of the imaging beam illuminates and reflectsfrom either region. The two bands in the image are the result of thedynamic balance between the signal returning from the two regions. Whenthe keyhole is more narrow, the shallower band of black pixelscorresponding to the liquid phase of the PCR at the top of the keyholeis seen more prominently. When the keyhole is wider, more of the bottomis illuminated, enhancing the deeper band black pixels corresponding tothe bottom of the keyhole is seen.

From this one can determine that the keyhole front wall is approximately25 μm ahead of the central axis of the processing beam and the back wallvaries but is approximately 75 μm behind the processing beam, for atotal keyhole length of approximately 100 μm. One can also calculate thefront wall slope by forming an imaginary line between the depth readingin FIGS. 4A and 4B taken at the same (or very nearly the same) instantin time. Other keyhole characteristics can be computed in a similarmanner.

Results accuracy can be improved by taking readings at more locationseither sequentially through multiple welds with different beamalignments, or dynamically by changing the beam alignment in real time.Other keyhole characteristics can be computed in a similar manner Aleading or trailing imaging spot could also allow measurement of the topsurface and bottom surface of the PCR at the same time. Notably, thesame approach can be applied to transverse keyhole dynamics just as wellas the longitudinal dynamics just discussed.

In some embodiments of the invention, one or more measurement beams willbe directed to multiple locations in and near the PCR in rapidsuccession (perhaps up to 100 kHz or more) by using a movable mirror(depicted as element 43 in FIG. 3) under imaging system control.

In some embodiments, imaging may be done by multiple measurement beamsdirected at multiple locations simultaneously. In some embodiments, thismay be accomplished by multiplexing the imaging beam with a geometricoptic such as a multi-faceted mirror and/or a prism.

Working Example—Second Experiment

In another experiment conducted using an apparatus based on FIG. 3, thelaser 17 was a 1070 nm, 1 kW IPG fiber laser which was connected to aLaser Mechanisms AccuFiber laser head 28. An imaging light source 21with a 840 nm wavelength was also connected to the laser head, reflectedoff a movable mirror 43 and through dichroic mirror 45 used to allow thelaser beam and the imaging beam to be directed towards the sample andclosely aligned. A sample of mild steel was moved at rates ofapproximately 1-6 m/min in a linear direction 46 while continuous wavekeyhole bead on plate welding was performed. Weld depths of up to 4 mmwere achieved. Trials were performed with the imaging beam 20 directedat various positions relative to the laser beam 16 such thatmeasurements within the phase change region (PCR) 30 were obtained.

The results demonstrated that by taking measurements at multiple pointswithin the PCR, information about the maximum weld depth, depthvariability, keyhole stability, and weld quality could be obtained. In aspecific example, the processing beam incident point is depicted at 41,and this moves with the sample, and the imaging beam is aligned to beincident at a point 42 which trails the processing beam incident point41 by about 25-75 μm. This particular approach reduced the verticalscatter of the maximum weld depth measurements substantially whencompared to imaging without such an offset. For larger keyholes or thosethat are deeper with different shapes, the optimum offset will bedifferent and can be determined experimentally. As before, a leading ortrailing imaging spot could also allow measurement of the top surfaceand bottom surface of the PCR at the same time. In some embodiments,imaging would be done at multiple locations in and near the PCR(depicted in FIG. 1 as imaging beams 20 a through 20 i) in rapidsuccession (perhaps up to 100 kHz or more). In some embodiments, this isachieved by using an electronically movable mirror (depicted in FIG. 3as 43) under imaging system control.

Working Example—Third Experiment

In a third experiment, an embodiment similar the one shown in FIG. 3 wasused to capture an interferometry signal from the PCR of a laser weld ata plurality of positions ranging from in front of the processing beam tobehind the processing beam. The result is shown in FIG. 11. Before theprocessing beam, the information may be used to determine the materialposition, surface quality, and geometrical compliance of the incomingmaterial. Additionally, scanning using, e.g., FIG. 3, element 43, onanother axis may locate a surface feature (e.g., a groove) which can beused to provide seam tracking capabilities before the processing beam.

As the imaging beam is located closer to the processing beam (labelled“capillary” in FIG. 11), it begins to sense the front wall and thebottom of the vapor component (i.e., keyhole) of the PCR. This permitsthe sensing of many keyhole dynamics such as depth, width, collapse,melt ejection, pore creation, and instability that may be importantquality indicators for the process.

As the imaging beam is located behind the processing beam, it begins tosense the melt pool surface. The rapidly changing height of this surfaceis an indicator of its liquid state since a solid material would notchange height so rapidly. Therefore, the length of the surfacedisruption is indicative of the melt pool length as shown in the figure.The length of this melt pool may be an important indicator of manyquality aspects for the process including proper mechanicalcontact/connection between parts that are to be joined. It may bepossible to assume that multiple welds that have the same melt poollength would share similar quality aspects and therefore thisinformation is useful for industrial quality control applications.

As the imaging beam is located further behind the processing beam, thesolid surface of the material is sensed. The roughness, height, anddepth of this surface may be an important quality indicator. Forinstance, many surface ripples indicate an unstable weld process thatmay have created high porosity and therefore compromised hermeticsealing and/or strength.

Furthermore, quantifying the amount of underfill along the surface ofthe seam is important information for determining the strength of theweld, its resistance to corrosion, and its compatibility with subsequentcoating processes such as priming and painting.

As the imaging beam is located even further behind the processing beam,the signal returning from the solid material surface becomes weaker.This is because oxidation of the surface has occurred, making it is lessreflective. This information is indicative of cooling rate and shieldinggas quality/coverage. Such information may be used, for example, tocorrect deficiencies in the shielding gas and/or reject parts.

Working Example—Fourth Experiment

In a fourth experiment, an embodiment similar to the one shown in FIG. 3was used to capture an interferometry signal from the PCR of a laserweld at a plurality of positions ranging from the left to the right ofthe processing beam. As shown in FIG. 12, the resulting data allowsestimation of a number of characteristics of the PCR including the meltpool width, the keyhole width, and keyhole depth.

The width of the melt pool may be an important indicator of many qualityaspects for the process, including proper mechanical contact/connectionbetween parts that are to be joined. It may be possible to assume thatmultiple welds that have the same melt pool length would share similarquality aspects, and therefore this information is useful for industrialquality control applications.

The width of the keyhole and asymmetries thereof may indicate the degreeof alignment of the process laser beam with joints of the material beingprocessed (e.g., in a butt or fillet weld) and may therefore be used toguide a seam-tracking process to better process the material.

Working Example—Fifth Experiment

In a fifth experiment, an embodiment similar to the one shown in FIG. 3was used. For this experiment, the laser 17 was a 1070 nm, 1 kW IPGfibre laser, and the laser head 28 was a Laser Mechanisms AccuFiber™head. An orthogonal pair of movable mirrors, actuated by galvanometers,was inserted between the collimating lens 22 and the dichroic mirror 43to enable rapid adjustment of the imaging beam alignment. A sample ofstainless steel plate was moved at rates of 5 m/min and 10 m/min whilecontinuous wave keyhole bead on plate welding was performed. Trials wereperformed with the imaging beam aligned coaxially with the processingbeam, as in previous experiments, and with the galvanometer-drivenmovable mirrors adjusted to place the imaging beam slightly ahead of orbehind the processing beam.

FIGS. 13A-13D are coherent images of keyhole laser welding with theimaging beam aligned ahead of or behind the processing beam. Each ofthese images contains data from two similar welds, performed in oppositedirections. These images are oriented with the top surface of thematerial towards the top of the image, and the zero delay point (at thebottom of the image) deep inside the material. In some cases, thesamples have warped with heat; this is visible as a tilt or curvaturealong the length of each of the two opposite-direction welds in eachimage.

In FIG. 13A, the welds were performed at 1000 W laser power with a feedrate of 5 m/min. The imaging beam was steered, using the galvanometers,to a focal spot 72 μm behind the processing beam in the first of the twowelds. The alignment of the imaging beam was held constant when thedirection of travel was reversed for the second weld in this image, sothe imaging beam was then 72 μm ahead of the processing beam. At highfeed rates, it is expected that the keyhole will lag slightly behind theprocessing beam. Aligning the imaging beam to a position behind theprocessing beam led to a stronger signal from the bottom of the keyhole;when the direction was reversed and the imaging beam was ahead of theprocessing beam, the bottom of the keyhole was not visible and strongsignals from near the top of the front wall of the keyhole wereobtained.

FIG. 13B shows the result of a similar experiment, identical to that ofFIG. 13A except for the feed rate (10 m/min instead of 5 m/min) and theimaging beam alignment set by the galvanometers (120 μm behind theprocessing beam instead of 72 μm). The first of the two welds in thisimage shows a strong reflection from the bottom of the keyhole, withweaker signals from near the top of the back wall of the keyhole. Whenthe direction was reversed for the second weld, and the imaging beam wasleading the processing beam, the signal was dominated by the reflectionfrom the surface of the melt pool ahead of the keyhole, with occasionalglimpses of the front wall of the keyhole.

FIGS. 13C and 13D illustrate the difference between fixing the alignmentof the imaging beam at a set position relative to the processing beam,and using the galvanometer-actuated mirrors to align the imaging beam ata set distance behind the processing beam given the current direction oftravel. The former case (FIG. 13C), the first weld of the pair,performed with the imaging beam slightly lagging the processing beam,yielded a good view of the bottom of the keyhole, but the secondweld—performed in the opposite direction—yielded a poor image. In FIG.13D, the alignment was adapted according to the direction of travel, andwelds in both directions yielded images of similar high quality.

While the effect of lagging/leading with the imaging beam is shown herewith linear welds, it is understood that a 2-axis galvanometer mirrorsystem (for example, a Scanlab AG Scancube™ 7 mm) allows this techniqueto be generalized to 2-D curvilinear welds. It is also understood thatthis approach may be generalized to “remote” welding heads such as theHighYAG RLSK™ (HIGHYAG Lasertechnologie GmBH, Germany), TRUMPF PFO™series (TRUMPF Laser-und Systemtechnik GmbH. Germany), and Scanlab AGIntelliWELD™. In one embodiment, illustrated in FIG. 14A for apre-objective scanner and in FIG. 14B for a post-objective or “F-Theta”scanner, this can be accomplished by using a collimating lens 22, a2-axis galvanometer scanner 401 and a refocusing lens 402, selected suchthat the output numerical aperture of this system substantiallyapproximates the supported numerical aperture of the remote head'scamera port, to image the fibre core onto the focal plane 403 where acamera sensor would normally be. By synchronizing the motion of this2-axis galvanometer scanner (and optionally the imaging system'sreference arm) with the remote head's motion then the optimal lagbetween imaging and process beams can be achieved. Programming of thisoptimal lead/lag can be done manually or automatically through SNRevaluation in iterative and/or dithering processes. Some embodiments ofthe invention may use a lookup table and/or memory of these optimallead/lag positions, as functions of the velocity and/or position of thefocal spot for various combinations of laser settings, that can beaccessed and updated offline or in real time.

EQUIVALENTS

Those of ordinary skill in the art will recognize modifications andvariations of the embodiments and examples described herein. It is to beunderstood that the disclosure may be practiced otherwise than asspecifically described herein, within the scope of the appended claims.

The invention claimed is:
 1. A system comprising: a beam delivery headconfigured to be coupled to a process laser that generates a materialprocessing beam that is applied to a sample location in a materialmodification process, wherein the beam delivery head is configured tofocus the material processing beam relative to the sample location andconfigured to scan the material processing beam relative to the samplelocation; an imaging system optically coupled to the beam delivery head,the imaging system configured to produce at least one imaging beam,wherein the imaging system is configured to direct a component of theimaging beam on a sample arm path through the beam delivery head to atleast one imaging beam position at the sample location and configured todirect a component of the imaging beam on at least one reference armpath to a reference reflective surface, wherein the imaging system is ofa spectral domain type or a swept-source type; wherein the imagingsystem includes an optical interferometer that produces aninterferometry output based on at least one sample optical path lengthto the sample location compared to a reference optical path length tothe reference reflective surface; and wherein the reference optical pathlength is configured to be adjusted in coordination with scanning thematerial processing beam relative to the sample location and incorrelation with a changing position of the focus of the materialprocessing beam as the processing beam is scanned.
 2. The system ofclaim 1 further comprising a controller configured to provide a controlsignal to control a focal distance of the material processing as thematerial processing beam is scanned, and wherein the imaging system isconfigured to automatically adjust the reference optical path length inresponse to the control signal to control the focal distance.
 3. Thesystem of claim 1 wherein the beam delivery head includes at least aportion of a sample arm path and at least a portion of the at least onereference arm path.
 4. The system of claim 1 wherein the referencereflective surface is adjustable for adjusting the reference opticalpath length.
 5. The system of claim 4 further comprising an adjustablefocal lens for focusing the processing beam and at least one actuatorcoupled to the adjustable focal lens and the adjustable referencereflective surface for adjusting the reference reflective surface incorrelation with adjusting the adjustable focal lens.
 6. The system ofclaim 4 wherein the beam delivery head includes at least a portion of asample arm path and at least a portion of the at least one reference armpath including the adjustable reference reflective surface.
 7. Thesystem of claim 6 wherein the beam delivery head further includes afocal lens for focusing the processing beam and an actuator for carryingthe focal lens and the reference reflective surface such that the focallens and reference reflective surface are adjustable together by theactuator.
 8. The system of claim 7 further comprising a controllercoupled to the actuator for automatically adjusting the actuator.
 9. Thesystem of claim 8 wherein the controller is configured to keep the focusof the processing beam a certain distance from a material surface. 10.The system of claim 1, wherein the imaging system is configured to scanthe imaging beam to provide seam tracking.
 11. A system comprising: abeam delivery head configured to be coupled to a process laser thatgenerates a material processing beam that is applied to a samplelocation in a material modification process, the beam delivery headincluding a scanning device configured to scan the material processingbeam during the material modification process, wherein the beam deliveryhead includes at least a portion of a sample arm path and at least aportion of at least one reference arm path including at least onereference reflective surface, wherein a reference optical path length isconfigured to be adjusted by adjusting the at least one referencereflective surface, wherein the beam delivery head is configured tochange the reference optical path length in coordination with scanningthe processing beam; an imaging system optically coupled to the beamdelivery head, wherein the imaging system is an optical coherencetomograph, the imaging system configured to produce at least one imagingbeam, wherein the imaging system is configured to direct a component ofthe imaging beam on the sample arm path through the beam delivery headto at least one imaging beam position at the sample location andconfigured to direct a component of the imaging beam on the referencearm path to the reference reflective surface in the beam delivery head,wherein the imaging system is of a spectral domain type or aswept-source type; and wherein the imaging system includes an opticalinterferometer that produces an interferometry output based on at leastone sample optical path length to the sample location compared to thereference optical path length.
 12. The system of claim 11 wherein thebeam delivery head is configured to change the reference optical pathlength in correlation with a changing position of a focus of theprocessing beam as the processing beam is scanned.
 13. The system ofclaim 11 wherein the beam delivery head is configured to adjust thereference reflective surface to change the reference optical path lengthin correlation with a changing position of a focus of the processingbeam as the processing beam is scanned.
 14. The system of claim 11wherein the beam delivery head further includes a focal lens forfocusing the processing beam and an actuator for carrying the focal lensand the reference reflective surface such that the focal lens andreference reflective surface are adjustable together by the actuator.15. The system of claim 14 further comprising a controller coupled tothe actuator for automatically adjusting the actuator.
 16. The system ofclaim 11, wherein the imaging system is configured to scan the imagingbeam to provide seam tracking.
 17. A method comprising: generating amaterial processing beam from a process laser; generating an imagingbeam from an imaging optical source, wherein the imaging optical sourceis of a spectral domain type or a swept-source type; focusing thematerial processing beam relative to a sample location; scanning thematerial processing beam relative to the sample location; directing acomponent of the imaging beam on a sample arm path to the samplelocation; directing a component of the imaging beam on at least onereference arm path; and adjusting a reference arm optical path lengthrelative to the sample location in coordination with scanning thematerial processing beam such that the reference arm optical path lengthcorrelates with a changing position of the focus of the materialprocessing laser beam as the processing beam is scanned.
 18. The methodof claim 17 wherein the reference arm optical path length is adjusted incorrelation with adjusting the focus of the material processing laserbeam.
 19. The method of claim 17 wherein the adjustable reference armpath includes an adjustable reference reflective surface, and whereinadjusting the reference arm optical path length includes adjusting theadjustable reference reflective surface.
 20. The method of claim 18wherein adjusting a reference arm optical path length in correlationwith adjusting the focus of the material processing laser beam includesadjusting a reflective surface actuator to synchronize adjustment of thereference reflective surface with actuation of a focal lens focusing theprocessing beam.
 21. The method of claim 18 wherein adjusting areference arm optical path length in correlation with adjusting thefocus of the material processing laser beam includes adjusting anactuator coupled to both a focal lens for focusing the processing beamand to the reference arm reflective surface.
 22. The method of claim 17further comprising scanning the imaging beam to provide seam tracking.